Plasma processing techniques are widely used for etching or depositing thin films on, or surface modification of, samples such as semiconductor substrates. As the density of semiconductor devices increases, plasma processing is being increasingly utilized because it can deposit films at lower temperatures than conventional techniques, can deposit a more conformal film and can also deposit and etch films in situ.
Recently, a number of triode plasma reactors have been designed. A typical triode plasma reactor, also referred to as a trielectrode plasma reactor, includes a reaction chamber for containing the sample to be processed and inlet and outlet ports for introducing a reactant into the chamber. Three electrodes are used to generate plasma in the chamber from the reactant.
Typically, the first electrode is a wall or other surface of the reaction chamber and is typically connected to ground potential, a floating potential or other fixed reference potential. The second and third electrodes are typically contained within the chamber and are electrically isolated from the chamber wall and from each other. Often one electrode is connected under or adjacent the sample holder and the other electrode is located above the sample holder, although many other configurations exist. At least one of the second or third electrodes is typically excited by a radio frequency (RF) voltage source, often at 13.56 MHz. An RF source may also excite the other electrode. Alternatively, often a low frequency or medium frequency source is used to excite the other electrode. Permanent or electromagnets are often provided in order to produce a magnetic field within the chamber to enhance plasma formation.
Many techniques have been devised for enhancing the performance of triode plasma reactors. For example, U.S. Pat. No. 4,464,223 to Gorin entitled Plasma Reactor Apparatus and Method describes a triode plasma reactor in which the first electrode is grounded, the second electrode is powered at a high frequency of about 13.56 MHz and the third electrode is powered by a low frequency of about 100 KHz. Because ions cannot respond to the main excitation frequency (13.56 MHz) the average ion energy impacting the sample surface is controlled by the lower frequency (100 KHz) voltage. At 100 KHz ions can transit the dark space region in response to the RF voltage applied. Unfortunately, the low frequency excitation also generates its own sheath voltage and DC bias and also interacts with the generation of reactive species, thereby limiting the performance of this design.
Another attempt to improve the operation of a triode plasma reactor is described in U.S. Pat. No. 4,585,516 to Corn et al. entitled Variable Duty Cycle, Multiple Frequency, Plasma Reactor. In the Corn et al. patent, the triode plasma reactor includes a grounded first electrode, a second electrode driven by a high frequency signal and a third electrode driven by a low frequency signal. A control apparatus is also provided to modulate the duty cycle of one or more of the signals so that the duty cycles of the signals overlap. In other words, the amplitude of the two voltage sources is modulated so that at least one of the signals has a duty cycle of less than 100%.
Another attempt to improve trielectrode plasma reactor performance is described in U.S. Pat. No. 4,863,549 to Grunwald entitled Apparatus for Coating or Etching by Means of a Plasma. In the Grunwald patent, the first electrode is grounded, the second electrode is connected to a high frequency RF signal generator and the third electrode is connected to a medium frequency RF generator. The medium frequency voltage consists of unipolar pulses that have the same amplitude for a predeterminable time interval. The number of ions impinging on the sample is thereby decoupled from the amplitude of the applied medium frequency voltage. This may contribute to the generation of undesirable reactive species.
Yet another attempt at providing an improved triode plasma reactor is described in U.S. Pat. No. 4,572,759 to Benzing entitled Triode Plasma Reactor with Magnetic Enhancement. In this patent, the first electrode is grounded and the second electrode is connected to a high frequency RF oscillator. The second electrode is connected to the high frequency oscillator through a variable phase delay. The phase of the RF power delivered to the third electrode relative to the second electrode may be adjusted by the variable phase delay from -180.degree. to +180.degree.. The optimum phase relationship depends upon the frequency of the RF energy chosen, the geometry of the chamber and the pressure and the nature of the gases used. When the optimum phase relationship between the wafer stage and the cathode is utilized, the processing rate is enhanced.
Notwithstanding the above improvements, there is still a need in the art for improved triode plasma reactors. In particular, for a given triode plasma reactor configuration there is a need to provide electrical parameters, other than frequency, amplitude, duty cycle, and phase lag between the two electrodes, which can be controlled to enhance the performance of the plasma reactor. A new dimension of control is needed to control the rate, selectivity and ion damage independent of the reactive species formation, to vary the ion current while maintaining the ion energy constant, and to provide uniform processing of a substrate which moves in the chamber during processing.